subprocess.PIPE is used to enable the parent process to communicate with
the subprocess via pipes, which mean all stdout and stderr messages are
captured and returned as part of Popen.communicate's result tuple.
In our case, we want to display the error messages on the console, so we
don't need to capture the output from stdout.
Example of a typical error message before this change:
```
Traceback (most recent call last):
File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 197, in <module>
sign_and_merge_tfm_bin(args.tfm_target, args.target_path, args.non_secure_bin, args.secure_bin)
File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 81, in sign_and_merge_tfm_bin
" secure binary, Error code: " + str(retcode))
Exception: Unable to sign musca_b1 secure binary, Error code: 1
```
Example of the error message after this change:
```
Traceback (most recent call last):
File "/mbed-os/tools/psa/tfm/bin_utils/wrapper.py", line 13, in <module>
import click
ModuleNotFoundError: No module named 'click'
Traceback (most recent call last):
File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 194, in <module>
sign_and_merge_tfm_bin(args.tfm_target, args.target_path, args.non_secure_bin, args.secure_bin)
File "platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/scripts/generate_mbed_image.py", line 80, in sign_and_merge_tfm_bin
raise Exception("Unable to sign " + target_name +
Exception: Unable to sign musca_b1 secure binary, Error code: 1
```
This is a significant improvement as now you can see what the reason for
the failure was.
Fixes: #14153
The target CYTFM_064B0S2_4343W is signed using `cysecuretools` which
depends on `imgtool` managed by `pip`. However, Arm Musca targets
requires a modified/wrapped version of the image tool copied from
trusted-firmware-m + MCUboot.
To avoid conflicts in the version of `imgtool` used, we should only
add the copied version of `imgtool` to python's system path for
Musca targets instead of doing it globally for all targets.
Robert Walton
Lingkai Dong
Vikas Katariya